1. Technical Field
The present invention relates to electrical circuits in general, and in particular to oscillator circuits. Still more particularly, the present invention relates to a single event effect immune oscillator circuit.
2. Description of the Prior Art
Oscillator circuits are commonly found in clock generation circuits, phase-locked loop circuits, and timing circuits. In environments having a relatively high-level of radiation, such as satellite orbital space, electronic devices that utilize oscillator circuits are more susceptible to single event effects (SEEs). These SEEs are typically caused by electron-hole pairs created by, and travelling along the path of, a single energetic particle as it passes through the oscillator circuit. Should the energetic particle generate a critical charge within the diffusion node of the oscillator circuit, a false pulse will be generated at the output of the oscillator circuit.
Referring now to the drawings and in particular to FIG. 1, there is illustrated a schematic diagram of a typical oscillator circuit according to the prior art. As shown, an oscillator circuit 10 includes invertors 11a-11d and a two-input NAND gate 12, all connected in series. In this implementation, the output of invertor lid also serves as the output for oscillator circuit 10. In addition, the pulse within oscillator circuit 10 can be turned off by de-asserting an enable input 13. For oscillator circuit 10, the diffusion nodes in each of invertors 11a-11d are very susceptible to SEEs. In order to reduce the susceptibility of oscillator circuit 10 to SEEs, very large n-channel and p-channel transistors are typically utilized to form invertors 11a-11d. While the effects of SEEs can be somewhat reduced with this approach, false pulses may still be generated by SEEs. Thus, the above-mentioned solution would not be acceptable for oscillator applications that are very sensitive to SEEs.
With reference now to FIG. 2, there is illustrated a schematic diagram of a SEE immune oscillator circuit, according to the prior art. Oscillator circuit 20 eliminates SEE transients by utilizing a redundancy scheme. As shown, oscillator circuit 20 includes three identical sets of oscillator circuits 20a-20c, each of which is similar to oscillator circuit 10 from FIG. 1. Output 23 from each of oscillator circuits 20a-20c are coupled to a voter 21. Voter 21 is designed to filter out any SEE transient pulses by allowing a pulse to occur at output 24 only if at least two of the input pulses at output 23 are identical. While oscillator circuit 20 having a voting scheme may eliminate SEE transient pulses at output 24, the circuit complexity is tripled due to the three-fold increase in the number of devices from oscillator circuit 10 of FIG. 1 and the added requirements of synchronization as well as phase issues among oscillator circuits 20a-20c. In addition, voter 21 itself may also be susceptible to SEEs.
Consequently, it is desirable to provide an improved oscillator circuit having a higher radiation tolerance. Radiation tolerance refers to the ability of an electronic device to withstand radiation without alteration of its electrical characteristics. An electronic device is said to be radiation tolerant if it can continue to function within specifications while experiencing SEEs.
In accordance with a preferred embodiment of the present invention, a single event effect immune oscillator circuit includes an odd number of logic circuit blocks connecting in series to provide a continuous pulse signal at an output of the oscillator circuit. Each logic circuit block has a first input, a second input, and an output. For a series of logic circuit blocks i, where i=1 to n (n is an odd number), the output of a logic circuit block i is connected to a first input of a logic circuit block i+1. The output of the logic circuit block i is also connected to a first input of a logic circuit block i+x, wherein x is an odd number greater than one and less than or equal to n.
All objects, features, and advantages of the present invention will become apparent in the following detailed written description.